52 DARWINISM 



song sparrow (Melospiza melodia), the fox-coloured sparrow 

 (Passerella iliaca), the swamp sparrow (Melospiza palustris), the 

 black and white creeper (Mniotilta varia), the water-wagtail 

 (Seiurus novseboracencis), in Turdus fuscescens and its allies, the 

 difference in the size of the streaks is often very considerable. 

 In the song sparrow they vary to such an extent that in some 

 cases they are reduced to narrow lines ; in others so enlarged 

 as to cover the greater part of the breast and sides of the body, 

 sometimes uniting on the middle of the breast into a nearly 

 continuous patch." 



Mr. Allen then goes on to particularise several species in 

 which such variations occur, giving cases in which two speci- 

 mens taken at the same place on the same day exhibited the 

 two extremes of coloration. Another set of variations is 

 thus described : " The white markings so common on the wings 

 and tails of birds, as the bars formed by the white tips of the 

 greater wing-coverts, the white patch occasionally present at 

 the base of the primary quills, or the white band crossing 

 them, and the white patch near the end of the outer tail- 

 feathers are also extremely liable to variation in respect to 

 their extent and the number of feathers to which, in the same 

 species, these markings extend." It is to be especially noted 

 that all these varieties are distinct from those which depend 

 on season, on age, or on sex, and that they are such as have 

 in many other species been considered to be of specific 

 value. 



These variations of colour could not be presented to the eye 

 without a series of carefully engraved plates, but in order to 

 bring Mr. Allen's measurements, illustrating variations of size and 

 proportion, more clearly before the reader, I have prepared a 

 series of diagrams illustrating the more important facts and 

 their bearings on the Darwinian theory. 



The first of these is intended, mainly, to show the actual 

 amount of the variation, as it gives the true length of the 

 wing and tail in the extreme cases among thirty specimens of 

 each of three species. The shaded portion shows the minimum 

 length, the unshaded portion the additional length in the 

 maximum. The point to be specially noted here is, that in 

 each of these common species there is about the same amount 

 of variation, and that it is so great as to be obvious at a glance. 



